Help checking Tube Amp Supply

[sonolink]

Hello

I have converted a Fender Pro Junior to p2p. Before firing it up for the first time I'd like to check that the Power Supply section is ok. This is the original schem:



So I thought of doing the following steps:

1-wire PT to mains through fuse and switch and check that the Tx is working and secondary voltages are correct.

2-wire Bridge Rectifier to transformer. Leave B+ unconnected and ground the other pole. Check voltage of B+



3-wire filter caps and bleed resistor and check voltages



Just to clarify,  this means running the Power Supply without any load.

4-I would then connect the Bias supply circuit to one of the Bridge Rectifier poles (where the TX is connected) and check its voltage to ground.



If everything checks ok I would then connect the rest of the amp's circuit.

Makes sense?
Thanks in advance for your help

Cheers
Sono

 

[ruffrecords]

Looks basically OK to me. It might be a tad cautious but that's no bad thing.

Cheers

Ian

 

[sonolink]

Ian

Thanks for your reply

One thing I just noticed is that the original Fender schematic states 132v at the Bridge Rectifier/Bias junction. I assume that the original TX would then deliver 264v on the HV winding.  If this is right, the TX I want to use is an asymmetrical non CT 0/275v.

Is the fact that it's not symmetrical a problem, since I guess that the Bias circuit would get a much higher voltage?

I hope this makes some sense

Cheers
Sono

 

[ruffrecords]

Not sure what you mean by asymmetrical transformer. Did the original power supply have a centre tapped transformer?

Cheers

Ian

 

[sonolink]

I don’t think so. At least it doesn't show on the schem and the original TX had only 2 windings (2xred leads and 2x green for heaters)

By asymmetrical I mean (maybe this is a wrong assumption by me) that my TX says "275-0" whereas on the heaters winding on the original schem says "3.3". I know that the heaters are 6.3 so I assumed that on that TX there's 3.3 on each lead and when connected them together they give 6.6v.

Since mine says "275-0" I thought that one side is ground and the other side 275v and since the bias circuit needs 132v I thought maybe that would be a problem.

But maybe I have the wrong idea about the TXs?
If you could clarify it would be great

Cheers
Sono

 

[ruffrecords]

OK, I understand. The 275-0 just indicates it is a single winding. A centre tapped transformer would be 275-0-275 or something similar. The heater voltages are measured relative to 0V or ground. You will see R35 and R36 connected across the 6.3VAC winding with their centre connected to 0V or ground. Hence each winding will measure half of 6.3VAC relative to ground

The 132VAC is probably just the equivalent AC voltage at that point. On positive cycles the voltage at that point rises to the reservoir capacitor  peak voltage which is 275VAC *1.414 = 388 volts. On negative half cycles it drops to 0V. So the peak to peak values is about 380 volts which is 135VAC.

Cheers

Ian

 

[sonolink]

Ian

Thanks a lot for your help

OK, I understand. The 275-0 just indicates it is a single winding. A centre tapped transformer would be 275-0-275 or something similar.

Ok. That makes sense

The heater voltages are measured relative to 0V or ground. You will see R35 and R36 connected across the 6.3VAC winding with their centre connected to 0V or ground. Hence each winding will measure half of 6.3VAC relative to ground

That's basically like creating a Central Tap right? Like emulating 2 windings?
But heaters need 6.3v. What's the purpose of R35/R36 to ground then? Why do we need a "central tap emulation"? Would it not be ok to connect the heaters directly without that?

The 132VAC is probably just the equivalent AC voltage at that point. On positive cycles the voltage at that point rises to the reservoir capacitor  peak voltage which is 275VAC *1.414 = 388 volts. On negative half cycles it drops to 0V. So the peak to peak values is about 380 volts which is 135VAC.

Oh I see! Now I understand that value

Thanks again!
Cheers

Ian
[/quote]

 

[sonolink]

Ok so I checked the TX. Alles gut

I built the Bridge Rectifier and all voltages seemed fine but when right when I switched off, the house fuse popped. The amp fuse is fine and nothing looks or smells funny.

Could that be because the filters aren't yet connected? This is only mains, fuse, switch, TX and diode Bridge Rectifier.
I'll wait to hear any reply before doing anything just in case

Cheers
Sono

 

[joaquins]

If your voltages are safety lower than your caps rates you should continue... About the PS, I don't know if I'd rely on the cap decider for my bias, little change in bias is quite a big change in the working conditions of the amp, and caps tolerances, drift and non linearities are quite over that, is not a bad start but I'd use a regulator after, I think a Zener is enough.

JS

 

[sonolink]

Joaquins
Thank you very much for your input

If your voltages are safety lower than your caps you should continue...

Well the caps are rated for 450v, and I did read 550v between the poles of the Bridge Rectifier that are NOT connected to the TX cables (the ones that connect to ground and caps positive rail). Incidentally,  550 is EXACTLY the double of the TX HV winding. ..

What do you reckon?

Cheers
Sono

 

[joaquins]

load it down with a resistor, big value, and maybe a ceramic cap rated for 1000V maybe, easy to find one at home if you are building a tube amp... maybe you are having bad reading because of charges or something, I don't know. You say your transformer is rated for 275V, right? so that 1.4 times you should get 385, maybe a little more because it's unloaded, about 400V or 410V probably would be right. Mostly is recommended to have caps rated about 30% over or more, so about 500V, one option is to use two caps in series, same value, and some drop resistors divider to ensure half voltage on each, you could start with that if you are not sure about something, then, once you are running your amp and you see it's working always below 400V you attach your cap as expected. The worst thing can happen is to blow a cap, but if they are expensive or hard to get won't be funny, if it happens on a 50V 470µF could be funny once in a while, it comes followed by s**t coming out of your mouth which is quite particular for caps blows or shock, ALWAYS LEAVE YOUR HEAD AWAY FROM IT,  they shouldn't but sometimes fly away or at least some oil, a friend get one into the ceiling which was up there for quite a long time as reminder... He heard a high pitch noise and get away from the circuit automatically, which save him for a bad hit of the cap.

JS

 

[ruffrecords]

The centre tap emulation is for the purpose of hum reduction. Sometimes the two resistor are replaced with a 100 ohm 5 watt wire-wound pot which can then be adjusted for minimum hum.

I think your capacitor rating should be fine. The 275VAC winding should produce a peak of no more than 275 * 1.414 Vdc = 388Vdc. 450V caps should be fine. When you connect the reservoir cap be sure to also add a resistive load in parallel to discharge the cap when you switch off. You will be surprised how long  an HT cap can hold its charge without a bleed resistor - which you will discover the day you accidentally put a couple of fingers across it.

Cheers

Ian

 

[sonolink]

Ian,

You are right indeed. I already had that experience but with a multimeter lead instead of my fingers! It was a radial cap and the tip slipped touching the other pole. Now you know why I'm a tad over cautious
It scared the c**p out of me and every time I think about it I sweat...

I thank you both you and Joaquins for the input

Ok so I wired up everything and all seems fine. No smoke nor pops
Did some readings to compare to the original schematic on my first post:
Mains: 234vac
Bridge Rectifier (where the TX is hooked): 250VAC
TP22: 160VDC / 350VAC
TP23 (bias): -10
TP24 (b+): 350VDC
TP25: 350VDC
TP26: 340VDC

A few thoughts and questions:

My voltages seem a bit higher than the original schematic. I guess it's because my mains are more than 220v.

There is something weird though: TP22 says 132VAC. I guess it's a typo since VAC should be the same in TP22 and TP24 so it should really read 132VDC, right?

R32 doesn't seem to do anything since TP25 and TP24 read the same voltage (350vdc). I checked both R32 and R33 and their resistance is correct. The power rating is both 3w.

Any ideas? Does it matter that I get 350 instead of 294 on TP25 and 340 instead of 232 on TP26?

One final thing: I wired a 470k bleed resistor after the first big cap. Its power rating is 0.6w (following the schem). On all other amps I've built the bleed resistor was usually a 2-5w one. May this have something to do with getting a higher voltage on TP25 and TP26?

I guess it's ok to add the rest of the amp?

Thanks a lot for your time and advice
Cheers
Sono

 

[sonolink]

Hi again
I finished building the amp but I get no sound. I can hear a very low hum in the speaker (VERY LOW) when switched on.
Any ideas?
Thanks
Sono

 

[ruffrecords]

Hi again
I finished building the amp but I get no sound. I can hear a very low hum in the speaker (VERY LOW) when switched on.
Any ideas?
Thanks
Sono

The high dc voltage reading you got was because there was no load on the power supply. These simple RC filter power supplies drop voltage when the load is applied. That's why you got the same reading at TP25 and TP24. TP22 and TP24 are not the smae and will be quite different when a load is applied.

If you get no sound with the amp connected then you need to start checking basic things.

1. Do the tube heaters glow?
2. Is there a voltage on the tube anodes?

if these are OK then you need to inject a signal and trace it stage by stage.

Cheers

ian

 

[sonolink]

Ian
Thanks a lot for your help :

This is all the info I can think of:

Mains 230vac
Low hum on speaker when switching on
Heaters winding voltages correct (6.3vac)
Heaters glowing on all 4 tubes
TX HV winding 245vac
B+ 345vdc (C15 anode to gnd)
V3 and V4 plate 346vdc (v3&v4 pin7)
C17 anode to gnd 338vdc
Across R32 8.6vdc
Across R33 68.5 vdc
EL84s cathode (v3&v4 pin3) 13.5vdc

I don't know how to calculate average plate current and dissipation on this amp. Not thay it would be of any use on finding why it's mute...

I'll try rewiring the input jack...
Any ideas are most welcome

Cheers
Sono

 

[sonolink]

I just rewired the input jack and tried different preamp and power valves. No go...grrrr
This must be something silly. And I'm such an idiot I can't figure it out!! Anyway I'd better go to bed and get back at it tomorrow....

Cheers
Sono

 

[PRR]

> B+ 345vdc
> V3 and V4 plate 346vdc

You say the plates sit 1V higher than what you are giving them?

I will assume the 1V difference is an error.

If the tubes are pulling current, the plate typically sits 1/4 to 3/4 of the B+.

I can't see ImageShack images, but with common Fender-like values, plate sits at 69% of B+.

If the tubes are NOT drawing current, then when you are not looking the plate sits AT the B+ voltage. When you do poke a voltmeter at it, meter loading causes a 1%(+) loading error, so plate seems to sit a few volts low of B+.

Why is the tube not drawing current?

Not glowing cathode (heater wiring error).

No path from Cathode back to B- (missing connections in cathode networks or ground path).

 

[PRR]

> don't know how to calculate average plate current and dissipation

You have low hum so this is not the likely problem.

However this is not budget science.

You have 8.6V across a resistor in the cathode return of the EL84s. (Which also says your grounding is OK to this point.)

I don't know the value of that resistor, but you do, and a quick V/R math will tell you the current through both EL84s.

Plate is at 346V and cathode is at 8.6V so plate-cathode is 337V.

337V times total cathode current is total Watts in both tubes.

Divide by 2 for one tube (ASS-uming they share fairly; not always true).

This is really Plate+Screen dissipation, not plate dissipation. For 6V6 you can almost ignore screen dissipartion. EL84 screens are hungry, may be sucking 10% of the total. So for a quick-pick, assume the plate dissipation alone is like 90% of the total. With modern EL84s aiming for MAXimum power output, this should be above 10 Watts but not over 15 Watts. (Book says 12W but that includes a margin for slop; guitar amps traditionally take all the margin and more.)

 

[sonolink]

if these are OK then you need to inject a signal and trace it stage by stage.

Could you please tell me how to do that? Could I do it holding a guitar cord (to create a signal) or it's better to use something else? What kind of meter do I need? DMM, oscilloscope? And where do I "poke"?

Thanks


Thanks for your post, PRR. Do you mean that the problem is in the tubes not pulling current? If so where should I look for the solution?

You have 8.6V across a resistor in the cathode return of the EL84s. (Which also says your grounding is OK to this point.)

I don't know the value of that resistor, but you do, and a quick V/R math will tell you the current through both EL84s.

It's a 2k7/3w metal oxide resistor.

Plate is at 346V and cathode is at 8.6V so plate-cathode is 337V.

337V times total cathode current is total Watts in both tubes.

Then current through both EL84s should be 3.185mA, which is way too low isn't it?

Thanks to all for your help
Cheers
Sono